In order to supply gases, for example humidified gases, to a patient or other person in need of such gases, flexible conduits have been used both from a gases supply to the patient (inspiratory conduit) and from the patient back to the gases supply (expiratory conduit). When the supplied gas is humidified, it is desirable to minimise the amount of condensation (known as "rain out") occurring on the inner walls of the conduit as this condensation soon accumulates, requiring regular draining. Some existing respiratory humidifier conduits incorporate heating wires to heat the conduit walls in an attempt to reduce condensation and also to assist in the control of the temperature of the humidified gases being delivered to the patient.
An example of a respiratory humidifier conduit incorporating a heating wire is disclosed in our prior United Kingdom Patent Application published as GB2284356A on Jun. 7, 1995. The heating wire disclosed is a looped heating element with the two free ends of the loop emerging from one end of the conduit for connection to a source of alternating voltage on the humidifier. This form of heated conduit where the heating wire lies in a random path along the bottom of the conduit has the disadvantage that gases passing through the conduit are not uniformly heated across the width of the conduit. In addition, the random nature of the wire's distribution allows for localised regions of the conduit walls to be at a temperature sufficiently low so as to allow condensation (or "rain out") to occur while other areas are heated excessively.
Some humidifier conduits have a heating wire wound around the outside of the conduit in an attempt to evenly apply heat to the conduit wall (both around the conduit and along the length of the wall) to overcome the problem of condensation. Examples of externally wound heated humidifier conduit may be seen in U.S. Pat. No. 4,686,354 to the BOC Group Plc and German patent publication number DE4244493 to Heinz Eilentropp. Both of these configurations, however, require the power drawn by the heating element to be sufficient to transmit heat through the conduit walls and into the gases. Accordingly, the power drawn by the heater wire is excessive as is the temperature of the wire. In addition, as heat from the heater wire must first pass through the conduit wall, the time taken to heat the gases is excessive, and the temperature of the outer surface of the conduit could possibly be high enough to burn a patient or care giver.